Polyethylene is the most widely used commercial polymer and can be prepared by a couple of different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the first method used to obtain polyethylene and continues to be a valued process with high commercial relevance for the preparation of low density polyethylene (LDPE).
A common set-up of a production line for preparing low density polyethylene comprises a polymerization reactor, which can be an autoclave or a tubular reactor or a combination of such reactors, and additional equipment. For pressurizing the reaction components, usually a set of two compressors, a primary compressor and a secondary compressor, is used. At the end of the polymerization sequence, a production line for high-pressure polymerization normally further includes apparatuses like extruders and granulators for pelletizing the resulting polymer. Furthermore, such a production line may comprise means for feeding monomers and comonomers, free-radical initiators, modifiers or other substances at one or more positions to the polymerization reaction.
A characteristic of the radically initiated polymerization of ethylenically unsaturated monomers under high pressure is that the conversion of the monomers is by far not complete. For every pass of the reactor or the reactor combination, only about 10% to 50% of the dosed monomers are converted in polymerizations in a tubular reactor and from 8% to 30% of the dosed monomers are converted in polymerizations in an autoclave reactor. The resulting reaction mixture usually leaves the reactor through a pressure control valve and is then may be separated into polymeric and gaseous components with the unreacted monomers being recycled. To avoid unnecessary decompression and compression steps, the separation into polymeric and gaseous components may be carried out in at least two stages. The monomer-polymer mixture leaving the reactor can be transferred to a first separating vessel, frequently called high-pressure product separator, in which the separation in polymeric and gaseous components is carried out at a pressure that allows for recycling of the ethylene and comonomers separated from the monomer-polymer mixture to the reaction mixture at a position between the primary compressor and the secondary compressor. At the conditions of operating the first separation vessel, the polymeric components within the separating vessel are in liquid state. The liquid phase obtained in the first separating vessel is transferred to a second separation vessel, frequently called a low-pressure product separator, in which a further separation into polymeric and gaseous components takes place at lower pressure. The ethylene and comonomers separated from the mixture in the second separation vessel are fed to the primary compressor where they are compressed to the pressure of the fresh ethylene feed, combined with the fresh ethylene feed and the joined streams are further pressurized to the pressure of the high-pressure gas recycle stream.
The polymerization process in a LDPE reactor is carried out at high pressures which can reach 350 MPa. Such high pressure may require special technology for the process to be handled in a safe and reliable manner. Technical issues in handling ethylene at high pressures are, for example, described in Chem. Ing. Tech. 67 (1995), pages 862 to 864. It is stated that ethylene decomposes rapidly in an explosive manner under certain temperature and pressure conditions to give soot, methane and hydrogen. This undesired reaction occurs repeatedly in the high-pressure polymerization of ethylene. The drastic increase in pressure and temperature associated therewith represents a considerable potential risk for the operational safety of the production plants.
A possible solution for preventing a drastic increase in pressure and temperature of this type involves installing rupture discs or emergency pressure-relief valves. WO 02/01308 A2, for example, discloses a specific hydraulically controlled pressure relief valve which allows a particularly fast opening of the pressure relief valve in case of sudden changes in pressure or temperature. It is technically possible to handle such thermal runaways or explosive decompositions of ethylene within the polymerization reactor, however these situations are highly undesirable since thermal runaways or explosive decompositions of ethylene within the polymerization reactor lead to a shut-down of the polymerization plant with frequent emission of ethylene into the environment and loss of production.
Another threat to the operational safety of high-pressure polymerization plants is the occurrence of leaks. Due to the high pressure difference between the interior of the polymerization reactor and the surroundings, even small fissures in a wall of high-pressure equipment may lead to an exit of a considerably high amount of the reactor content resulting in locally high concentrations of combustible hydrocarbons in a short time period. On the other hand, in the case of larger leaks, the available time for reacting is extremely short. Depending on the size and the position of the leak, the leakage rate of combustible or explosive gases may be extremely high.
Furthermore, in processes for preparing ethylene polymers at high pressure, the reaction mixture may comprise a supercritical composition comprising monomer and polymer. After a leakage of such a reaction mixture into the atmosphere, small polymer particles are formed which are subject to electrostatic charging. Consequently, there is an enhanced probability for an ignition after an explosive gas cloud has developed after an escape of the reaction mixture.
It is common in chemical and petrochemical plants to monitor the surroundings of such plants with respect to the leakage of combustible gases by gas detectors. Gas detectors are devices that detect the presence of gases in an area, often as part of a safety system. This type of equipment is commonly used to detect a gas leak and can interface with a control system so a process can be automatically shut down. A gas detector can also sound an alarm to operators in the area where the leak is occurring, giving them the opportunity to leave. Gas detectors can be used to detect combustible, flammable and toxic gases, and oxygen depletion. Common gas sensors include infrared point sensors, ultrasonic sensors, electrochemical gas sensors, and semiconductor sensors. More recently, infrared imaging sensors have come into use. These sensors are used for a wide range of applications and can be found in industrial plants, refineries, waste-water treatment facilities, vehicles, and homes.
After detection of a leakage of monomers or reaction mixture, it is possible to interrupt the polymerization process and depressurize the polymerization plant or parts of the polymerization plant. However, until the leakage is detected a critical amount of combustible or explosive gases may have been released and the leakage does not end until the plant is fully depressurized.
EP 2 732 852 A1 discloses a method to mitigate the consequences of a vapor cloud explosion due to an accidental release of a flammable gas in an open area, in which a flame acceleration suppression product is released in a defined hazardous area at a rate that is determined as a function of the volume of the hazardous area. EP 2 732 852 A1 describes that the flame acceleration suppression products is preferably a dry powder of a radical capturing salt. The powder may be supplied by a carrier gas, e.g. nitrogen.
There is a need to overcome the disadvantages of the prior art and provide a process which include fast and effective steps to reduce the probability that a leaked gas cloud could explode within an enclosed area and to mitigate the negative effects of explosions which may occur.